Conventional electrical discharge machining (EDM) wire equipment comprises a head assembly mounted on a machine tool body and a numerically controlled positioning table. The wire equipment contains a spool assembly on which a fine wire is held taut between two spools. The drive spool moves the wire at a fixed rate from one spool to another with the useable work length between the two spools being about 4" to 8" depending on the work height to be cut. The cutting rate depends upon the wire size and the amperage carried by the wire, plus the feed rate set for the depth of the cut. Deionized water is normally used in the cutting zone as a dielectric, however, hydrocarbon oils can also be used. As with all EDM operations, the electrode, which in this case is in the form of a wire, produces an overcut such that an 0.008" diameter wire may produce an overcut of 0.001" per side and form a 0.010" slot.
In the extrusion forming of ceramic honeycomb structures used in catalytic converters for internal combustion engines, it is necessary to form extrusion dies with substantially long, thin slots which are not only straight and extend across the face of the die, but which also have parallel sidewalls, are burr-free, and have consistent surface finish. In addition, such long, thin slots should have a length greater than about 31/2", a depth in excess of about 0.10" and a width spot greater than 0.012".
Known EDM wire processes, such as disclosed in U.S. Pat. Nos. 2,526,423; 4,205,213; and 4,233,486, are all capable of producing slots, however, most machine tools and their processes have a limited effective depth of slot which can be cut with a given wire diameter over a length of 41/2" in the surface of a workpiece. That is, for example, the difficulty encountered with known EDM wire processes of flushing out the eroded particles from within small slots larger than 0.060" in depth during the cutting process, limits the effective depth of the cut. As the wire cutting process stalls due to a build-up of eroded particles, the slot width becomes materially enlarged or "overcut" as it is known at such depth, thus producing uneven slots and wire breakage.
The present invention overcomes the problems encountered with the known EDM wire processes of limiting slot depth of 0.060" and wire breakage, and permits the repeatable formation of a plurality of long, thin slots having parallel side walls with consistent surface finish and depths in excess of 0.060" and length of 8" or greater.